The invention relates to optical fibers for use in communication systems and, more specifically, to devices which reduce mode interference in the fiber.
Propagating a high order mode in optical fiber transmission systems can have advantages over propagating a fundamental or basic mode. For example, propagating a high order mode can improve the overall performance of transport systems. One way performance is improved is due to the choice of fiber that will support the propagation of the high order mode. A significant advantage of this high order mode fiber is that it can be designed to have strong negative dispersion and high effective areas. Therefore, it can be used to compensate for chromatic dispersion. There is a drawback to these high order mode fibers, however. Propagating a high order mode can generate interferometric noise due to mode coupling in the fiber. To substantially reduce this noise, it would be advantageous to propagate a single high order mode.
Various methods have been suggested for transforming or coupling light energy in the fiber from one mode to another different mode. For example, a long period fiber grating may be used to transfer energy from one mode to another mode. Unfortunately, this method can also transform some of the light energy from one mode to other undesirable modes. Other methods can have the same undesired results.
Energy transfer or coupling from the desired high order modes to undesirable modes can also occur due to inhomogeniety of the high order mode fiber. These inhomogenieties can occur in the manufacturing process. Inhomogenieties can also appear due to imperfect splicing of the fiber, periodic bending (micro bending), and scattering mechanisms (i.e., Riley scattering), for example.
The result of imperfect mode transformation and mode coupling in the fiber is that undesirable modes will propagate in the fiber. These modes can interfere with the desired mode through a process called multipath interference (MPI). MPI causes significant reduction in signal quality by distorting its phase and amplitude. Therefore, in order to realize reasonable signal quality, the ratio between the energy transmitted through the undesired modes to the energy transmitted through the desired mode should be below 1:10000 or approximately 40 dB. This ratio should be maintained for any length of high order mode fiber being used in order to maintain reasonable signal quality.
It is therefore desirable to suppress the undesired modes in a high order mode fiber in order to improve the signal quality in the fiber.
The invention relates to an optical waveguide for attenuating undesired modes. In one embodiment, the waveguide includes either an absorbing or scattering annulus disposed substantially within a core of the waveguide. In another embodiment, the absorbing or scattering annulus is concentric about the core of the waveguide. In another embodiment, an optical fiber is used as a waveguide and the optical fiber supports the LP02 mode. In still other embodiments, the width and the radius of the absorbing or scattering annulus are predetermined so as to attenuate undesired modes. In another embodiment, the annulus is disposed at a radial position corresponding to a region in the core in which the desired mode has substantially no energy. Other embodiments include a plurality of absorbing and/or scattering annuli. In another embodiment, the absorbing or scattering annulus includes titanium. In still another embodiment, the absorbing or scattering annulus includes boron. In still another embodiment, the absorbing or scattering annulus includes Erbium. In still other embodiments, the annulus includes any suitably absorbing material. In other embodiments, the annulus includes any suitably scattering material. In still another embodiment, the annulus includes any suitably conductive material dopant.
In another embodiment, the optical waveguide includes a multimode waveguide having a core and supporting a plurality of modes, the core having a core radius and a core refractive index. The optical waveguide further includes a sharp change of refractive index within the core. The refractive index discontinuity in another embodiment occurs at a radial position which is predetermined so as to attenuate a desired mode to a lesser degree than any other mode in the plurality of modes. The sharp change in index of refraction in the core, in one embodiment, comprises a discontinuity in the refractive index profile of the core.
In another embodiment, the optical waveguide includes a multimode waveguide having a core and supporting a plurality of modes, the core having a core radius and a core refractive index. The optical waveguide further includes a region of increased refractive index disposed within the core, and the region of increased refractive index step has a refractive index which is greater than the core refractive index and the radial position of the region of increased refractive index is predetermined so as to attenuate a desired mode to a lesser degree than any other mode in the plurality of modes. Another embodiment includes a plurality of regions of increased refractive index within the waveguide.
In another embodiment, the optical waveguide includes a multimode waveguide having a core and supporting a plurality of modes, the core having a core radius and a core refractive index. The waveguide further includes a region of decreased refractive index disposed within the core, and the region of decreased refractive index has a refractive index which is less than the core refractive index and the radial position of the region of decreased refractive index is predetermined so as to attenuate a desired mode to a lesser degree than any other mode in the plurality of modes. Another embodiment includes a plurality of regions of decreased refractive index within the waveguide.
In one embodiment, the desired mode is a high order mode. In another embodiment, the desired mode is an even high order mode. In yet another embodiment, the desired mode is the LP02 mode. Other embodiments include combinations of annuli. These combinations of annuli can include, for example, absorbing annuli, scattering annuli, annuli comprising conductive dopant material, regions of increased refractive index, and regions of decreased refractive index or any combination thereof.
In one embodiment of the invention, a method for attenuating undesired modes includes providing a multimode waveguide having a core, transmitting an optical signal having a plurality of modes through the waveguide, and absorbing the desired mode to a lesser degree than any other mode in the plurality of modes.